1. Field of the Invention
The present invention relates to a metal bearing for use in a high speed motor and to a manufacturing method there for. More particularly, the present invention relates to a method of forming grooves in an inner circumferential surface of the bearing to improve various properties thereof.
2. Description of the Prior Arts
In recent years, requirements for low vibration, high rigidity, low friction, and a long life span have been made on the performance of a bearing for use in a high speed motor for driving a disk in an audio-visual system using a digital disk as a medium, such as OA or AV, or in a computer memory device. To satisfy such requirements at low cost, a so-called kinetic pressure bearing has been used in a motor. In a motor having such kinetic pressure bearing at the center thereof, a lubricant is provided between a rotating shaft of the motor and the inner circumferential surface of the metal bearing disposed in opposing and closely spaced relation to the rotating shaft. In this constitution, a circumferential flow of the lubricant caused by the rotation of the shaft is changed into a diametral flow by the kinetic pressure bearing such that the rotating shaft is enhanced rigidity.
In accordance with a conventional method of manufacturing a kinetic pressure bearing, however, grooves are formed in either of the shaft and the metal bearing by etching or cutting process, so that the process becomes complicated and costly. Accordingly, the resulting bearing are not appropriate for use in a low-cost motor in spite of its excellent performance.
The present invention has been made in view of the foregoing problem and it is therefore an object of the present invention to stably provide a high-quality bearing with improved controllability which is capable of performing a smooth movement with low vibration, low noise, and high rotation accuracy in a short time and at a low cost.
To attain the foregoing object, in a method of forming ridges in a metal bearing as a kinetic pressure bearing according to the present invention, in which the metal bearing is located at a center of a rotary electric motor, a lubricant is provided between a rotating shaft and an inner circumferential surface of the metal bearing disposed in opposing and closely spaced relation to the rotating shaft, and a circumferential flow of the lubicant caused by the rotation of the shaft is changed into a diametral flow such that the rotating shaft is enhanced rigidity, the method comprising the steps of: inserting a mandrel having grooves of a desired configuration formed in an outer surface thereof into the inner circumferential surface of the metal bearing; and reducing an outer diameter of the metal bearing to form projecting ridges for generating dynamic pressure in the inner circumferential surface of the metal bearing.
In the method of forming ridges in a metal bearing according to the present invention, when the bearing has an outer diameter D and an inner diameter d, a reducing jig has an inner diameter A, the mandrel to be inserted into the inner circumferential surface of the bearing has a diameter B, (Dxe2x88x92A)/D is set to be 0.005 to 0.035, and (dxe2x88x92B)/d is set to be equal to or smaller than 0.005, provided that D greater than A and d greater than B are satisfied.
In the method of forming ridges in a metal bearing according to the present invention, when the bearing has an outer diameter D and a housing for holding the bearing has an inner diameter C, (Dxe2x88x92C)/D is set to be 0.005 to 0.035, provided that C less than D is satisfied, the mandrel is inserted into the inner circumferential surface of the metal bearing, a reducing process is performed simultaneously with the insertion of the metal bearing into the housing, and the ridges for generating dynamic pressure are provided in the inner circumferential surface of the metal bearing.
The method of forming ridges in a metal bearing according to the present invention is characterized in that a projecting portion provided at the inner circumferential surface of the metal bearing has a height in the range of 0.001 to 0.005 mm.
The method of forming grooves in a metal bearing according to the present invention is also characterized in that the movement of the mandrel used to machine the inner circumferential surface of the bearing is unfixed and free in a radial direction but fixed only in an axial direction.
The present invention provides a metal bearing formed in accordance with the aforesaid method. The metal bearing includes two separate and distinct metal bearings or two integrally formed metal bearings to arrange the ridges for generating dynamic pressure in two or more rows, and to provide a difference of 0.01 mm or more between the respective inner diameters of the bearings having the ridges in different rows. Furthermore, a sintered alloy is used as a material for composing the bearing. In addition, the ridges for generating dynamic pressure formed in the inner circumferential surface of the bearing are provided in a single-row or plural-row configuration.
According to the present invention, a mandrel having grooves of a desired configuration (herringbone, vertical, spiral, and like configurations) in the outer circumferential surface thereof and having an outer diameter slightly smaller than the inner diameter of a metal bearing is inserted into the inner circumferential surface of the metal bearing followed by reducing the outer diameter of the metal bearing by inserting the metal bearing into a reducing jig or housing having an inner diameter slightly smaller than the outer diameter of the metal bearing, thereby causing plastic deformation of metal and forming desired ridges in the inner circumferential surface of the metal bearing. When the bearing has an outer diameter D and an inner diameter d, the reducing jig or housing 7 has an inner diameter A, and the mandrel to be inserted into the inner circumferential surface of the bearing has a diameter B, the dimensional relationship between the individual components is established such that (Dxe2x88x92A)/D is in the range of 0.005 to 0.035 and (dxe2x88x92B)/d is equal to or smaller than 0.005 provided that D greater than A and d greater than B are satisfied. The foregoing method enables simple and stable formation of ridges for improving the performance of the bearing and thereby provides a high-performance bearing at low cost.